Internal erosion by suffusion can change dramatically the constitutive behavior of granular materials by modifying the fabric of granular materials. In this study, the effect of an internal fluid flow on granular materials is investigated at the material point scale using the numerical coupling between a Discrete Element Method (DEM) and a Pore-scale Finite Volume (PFV) coupling scheme. The influence of the stress state and the hydraulic loading (direction and intensity) on the occurrence of grain transport in dense widely graded granular samples is thus investigated and interpreted in terms of micromechanics. In particular, it is shown that, grain transport is increased when the macroscopic flow direction is aligned with the privileged force chain orientation. The stress induced microstructure modifications are shown to influence the transport distances by controlling the number of rattlers.